Charged particle beam systems, such as focused ion beam (“FIB”) systems, have found many applications in various areas of science and industry. For example, in the semiconductor industry, FIB systems are used for integrated circuit (“IC”) probe point creation, circuit editing, failure analysis, and numerous other applications. A FIB tool typically includes a particle beam production column designed to focus an ion beam onto the IC at the place intended for the desired intervention. Such a column typically comprises a source of ions, such as Ga+, produced from liquid metal. The Ga+ is used to form the ion beam, which is focused on the IC by a focusing device comprising a certain number of electrodes operating at determined potentials so as to form an electrostatic lens system. Other types of charged particle beam systems deploy other arrangements to produce various charged particle beams.
Successful use of a FIB tool depends, in varying degrees, on obtaining high resolution images of the IC or other sample. The images allow the user to view the IC during use of the FIB tool. Various phenomena, such as secondary electrons, ions, neutrons and photons, are available for monitoring FIB editing and generating images. Secondary electrons, in particular, are emitted as a result of the ion beam incident upon the IC. A common type of the secondary electron detector (“SED”) in FIB systems involves an Everhart-Thornley type design using scintillator. A scintillator typically includes a thin glass disk coated with a phosphor that converts energy from secondary electrons into light photons. The scintillator collects some of the secondary electrons emitted from the IC and generates photons responsive to the secondary electrons. In the photomultiplier tube, each photon generates multiple electrons, which are then used to generate an image.
Different material characteristics provide different numbers of secondary electron emissions. For example, with regard to an IC, a dielectric emits substantially less electrons than a metal. Typically, the greater the number of electrons, the brighter the image. Lack of electrons provides a dark image. By rastering the ion beam in a grid-like pattern, the contrast differences are used to generate an image of the target portion of the IC. To generate a clear and accurate image, the secondary electron collection efficiency is an important aspect of any FIB tool. Oftentimes, a large portion of the secondary electrons are emitted away from the SED, making collection difficult.
A practice referred to as “circuit editing” is one example of a use for a FIB tool. Circuit editing involves employing an ion beam to remove and deposit material in an IC with precision. Through removal and deposit of material, electrical connections may be severed or added, which allows designers to implement and test design modifications without repeating the wafer fabrication process. Due to the small scale of the circuit editing process, its success depends strongly on FIB image quality, which, as discussed above, is directly linked to the number of secondary electrons detected by the secondary electron detector.
Circuit editing success also depends on a process referred to as “endpointing.” Endpointing involves determining when to stop the FIB milling operation. It is the objective of the operator to stop the milling process at an interface at which the secondary electron signal changes. In one example, endpointing involves detecting the secondary electron signal as the ion beam drills down into the IC. The emission volume is dependent on the material the beam is milling. As mentioned above, metal emits a greater number of secondary electrons than a dielectric. Thus, if the electron emission characteristics are detectable, then boundaries between dielectrics and metals are detectable. The current used to generate an ion beam determines the power of the beam and the size of the hole generated by a beam. As vertical interconnects in an IC get laterally smaller, the ion beam etching current must be decreased. Besides reducing the hole size, the secondary electron signal also decreases. Thus, endpointing becomes more difficult as the secondary electron emission decreases. Further, as the depth of a milling operation increases, the number of secondary electrons that escape the hole becomes less. As such, with less secondary electrons to detect, high collection efficiency becomes more important.
One way to improve the collection efficiency of a SED involves the application of a high voltage (˜10 kV) to the scintillator surrounded by a grounded cap to produce a collection electric field that attracts the secondary electrons. One such system is described in U.S. Pat. No. 6,630,667 titled “Compact, High Collection Efficiency Scintillator for Secondary Electron Detection,” to Wang et al. and issued Oct. 7, 2003, which is hereby incorporated by reference herein. Through the generation of such an electric field, some of the secondary electrons initially emitted in directions away from the SED, are attracted to the scintillator thereby increasing the collection efficiency. Such a system has been successfully employed in Credence Systems Corporation's IDS P3X® FIB system.
However, in FIB systems where it is difficult or impossible to introduce such an collection field proximate the sample, improving secondary electron collection efficiency and its attendant image improvements remains a problem.
Moreover, in some instances, when a high voltage is applied to the SED, the SED behaves as a focusing lens causing secondary electrons to strike the scintillator disc within a very small discrete spot. Over time, a “burn” spot will result with much or all of the phosphor burned from the scintillator disc, leading to reduced detection and a reduced lifetime of the disc. The lifetime of the scintillator disc is further shortened when too many secondary electrons strike the disc as is the case when the primary ion beam current is high. In many instances, a less powerful ion beam might be employed for a particular operation. However, because of the need to detect secondary electrons, a higher beam current is employed to cause the emission of a greater number of secondary electrons.
It is with this background in mind that the inventors developed the various embodiments of the invention described below.